There are a variety of methods and apparatus which may be used to heat a number of thermoplastic materials for welding and other thermal bonding operations. A number of such methods and apparatus are discussed in various editions of Modern Plastics Encyclopedia, McGraw Hill, New York, N.Y.
These thermal bonding and welding techniques have various limitations. Some require a high degree of operator skill and experience (e.g., hot gas welding). Some are limited to certain materials or shapes (e.g., friction welding, spin welding, and R.F. dielectric heating). Others require additions to the plastic (e.g., the addition of metal particles to permit induction heating of the plastic).
In contact heating or welding, the thermoplastic parts to be welded are normally pressed against a heated die or tool, usually under some pressure. After the parts are heated, the die or tool is removed and the heated parts are pressed together to form the bond or weld. Alternatively, a die heated directly, e.g., by resistance heating, might be configured to clamp the parts together. In any case, the heated tool is removed from the thermoplastic parts when sufficient heating has been achieved. When such hot dies or tools are removed from what are often molten thermoplastic parts, the parts and/or the weld itself may be disturbed, and this may tend to weaken the weld and cause a deterioration in appearance or strength.
Thus, in contact heating, the thermoplastic parts to be adhered, welded, or sealed are initially pushed under pressure against a heated tool until the desired consistency is achieved. The parts and the heated tool are then separated, the tool removed, and the heated parts pressed together to effectuate the desired bonding. Such techniques require precise timing and temperature control and, typically, also produce a significant weld bead which may be undesirable in finished products.
The quality of a bond or weld of thermoplastic material is related to proper heating and cooling of the parts being mated, among other factors. If the weld site is underheated, inferior welds may be produced, and/or the weld cycle lengthened unduly. Conversely, if the die is too hot, the thermoplastic parts or materials to be welded can be overheated, which can also produce inferior bonds or welds. Similarly, a poor quality weld can result from improper cooling of the welded parts. This can occur, for example, if the weld is disturbed, such as by removing dies or tools from the welded or bonded parts prematurely before the thermoplastic material has had an opportunity to cool and set.
Equipment for achieving thermoplastic welding, such as heaters and associated circuitry, is known but is often expensive. Costs easily escalate if it is necessary to provide multiple installations of equipment, such as would be used for many automated techniques. Furthermore, in automated production, it is often necessary to be able to adjust process parameters for different parts and for different conditions, which may be difficult and time-consuming.
It would be desirable, therefore, to provide a method and apparatus for welding thermoplastic parts, which could result in improved quality and uniformity of the welded parts, which would be compatible with improved production capabilities, which would be readily adaptable to automated production, and which could be achieved without markedly increasing operating costs and capital investment.